This is an idea that just came to mind. It features Ron largely, which is strange as I usually don't like him. He's just too jealous (especially in combination with his inferiority complex) and left Harry hanging once too often in my opinion. I'm usually pretty forgiving, but the way he had been written, even I would have eventually taken a step back from that friendship.
Still, despite that leading to me reading a lot of fics with him in a "bad" role (him betraying Harry, Ron bashing and so on...), I don't tend to mind if he's written as Harry's best friend in a good way. Especially when he admits to his flaws - namely the jealousy - and works on getting over it.
The idea that I had features Ron betraying Harry, but not in the usual way (at least I hope so). It starts with Harry thinking about Ron becoming a Death Eater, his first kill having been Dumbledore (that they know about). Now, they've got him captured and question him. Through this, Harry finds out that it all leads back to Dumbledore.
Dumbledore had been worried because Harry didn't go to adults for help, so he used Ron's jealousy to make him spy on Harry. As Dumbledore festered that jealousy it slowly started turning to hate as Ron never learned how to get over it as he got older, but rather learned that it was OK to be jealous because Harry wasn't deserving. This hate eventually became to strong that he didn't want to be on the same side as Harry any more (guess Dumbledore miscalculated there) and Dumbledore dies the die he realises what he's done. The initial friendship between Harry and Ron had been genuine.
Now, Harry somehow (need to think of a good why yet) goes to the past and is now faced with a difficult choice: How does he save Ron from being used so much that he starts hating his best friend? How can he make sure that his best friend will be able to become the person he was meant to be and not some distorted version of it?
So, in effect the entire story is going to be about the friendship between Harry and Ron as well as Ron's development as he grows up and realises that not everything is as (un)important as he thinks it to be.
Just like another Ron centric idea I once had, I'm not sure if I can really do it justice because I don't quite like him. Especially since Harry knows what Ron is capable of and will always have some kind of inner conflict about it. Betrayal can never really be forgotten.
My previous idea didn't feature any kind of betrayal, but merely started as a "What If" for CoS. The what if in question was, "What if Ginny died in the Chamber of Secrets?" which logically left me with Harry being on the same side as Ron when the ceiling caves in and they both have to move through.
This ultimate brush with death that can't be undone effectively shows Ron how important family is and how easily it can be taken from him. This also shows him how Harry must feel, giving him a new understanding of Harry's situation. After all, what use is all of Harry's money if he doesn't have a family that loves him?
But as I said, I'm not quite sure if I can do those ideas justice because of my predisposition when it comes to Ron.